Because of shortcomings of metallic plates, bioabsorbable, polymeric plates have been developed for fracture fixation in bone surgery. E.g. elongated, bioabsorbable, six-hole plates were developed by Eitenmüller et al. for orthopaedic animal studies (European Congress on Biomaterials, Abstracts, Instituto Rizzoli, Bologna, 1986, p. 94). However, because of inadequate strength, some of the fracture fixation plates were broken in animal experiments.
U.S. Pat. No. 5,569,250 describes a biocompatible osteosynthesis plate operable for being enhanced in a substantially secured relation to a plurality of adjacent bone portions. The osteosynthesis plate is in a first configuration at a first thermochemical state and is operable to be converted to a second thermochemical state so that it may be deformed prior to fixation.
The first thermochemical state is typically room temperature (operation room conditions) and the second thermochemical state is typically an elevated temperature above Tg (glass transition temperature) of the polymer material (e.g. for polylactides between 50-60° C.). Accordingly, the plates of U.S. Pat. No. 5,569,250 must be changed from the first thermochemical state to the second thermochemical state, to be shaped (deformed) and thereafter they must be changed again back to the first thermochemical state prior to fixation. Because the thermal conductivity of polymeric materials is poor, the conversion of material to a second temperature is a slow process. Therefore, the clinical use of plates of U.S. Pat. No. 5,569,250 is tedious, slow and complex especially if the surgeon must shape the plate several times to make it fit exactly to the form of the bone to be fixed.
K. Bessho et al., J. Oral. Maxillofac. Surg. 55 (1997) 941-945, describe bioabsorbable poly-L-lactide miniplate and screw system for osteosynthesis in oral and maxillofacial surgery. Also these plates must be heated by immersion in a hot sterilized physiologic salt solution or by the application of hot air until they become plastic and can only then be fitted to the surface of the bone.
EP 0 449 867 B1 describes a plate for fixation of a bone fracture, osteotomy, arthrodesis etc. said plate being intended to be fixed on bone at least with one fixation device, like screw, rod, clamp or corresponding, wherein the plate comprises at least two essentially superimposed plates, so as to provide a multilayer plate construction, so that the individual plates of said multilayer plate construction are flexible so as to provide a change of form of said multilayer plate construction to substantially assume the shape of the bone surface in the operation conditions by means of an external force such as by hand and/or by bending instrument directed to said multilayer plate construction, whereby each individual plate assumes the position of its own with respect to other individual plates by differential motion along the coincident surfaces. The different plates can be manufactured of materials oriented and/or reinforced in different directions.
Although the said multilayer plate fits even the curved bone surface without heating of individual plates, the clinical use of multilayer plate is tedious, because the single plates easily slip in relation to each other before fixation. Additionally the thickness of multilayer plate system easily becomes too thick for cranio maxillofacial applications, causing cosmetic disturbance and increased risks for foreign body reaction.
EP 0 987 033 A1 describes a biodegradable and bioabsorbable implant material wherein its shape after deformation within ordinary temperature range can be fixed and maintained so that its shape can be easily adjusted at the site of operation, and it has substantially no anisotropy in view of strength. Particularly, it provides an implant material which can effect deformation such as bending or twisting within ordinary temperature range and has a shape-keeping ability to fix and maintain the shape after deformation as such, wherein molecular chains, domains of molecular chain assembly or crystals of the polymer are oriented along a large number of reference axes having random axial directions.
The small orientation units of molecular chains, domains of molecular chain assembly or crystals of EP 0 987 033 A1, form a non-continuous, random (non-directed) reinforcement into the material, analogous with the random short-fiber or whisker reinforcement. It should be advantageous to have in the oriented plate material a multiaxial continuous, directed, long-fiber like orientation, while long reinforcement units give usually better mechanical property combination for the material than short ones.
Random axial directions of oriented units could make the material structure unfavorable to resist shear (plate cutting) loads, which originate from the tendency of bone fragments to move (glide) in relation to each others in the direction of fracture plane (typically in the plane perpendicular to the flat surface and long axis of the plate).
Even if the implant material of EP 0 987 033 A1 can be bent or twisted within ordinary temperature range, it is manufactured with a complex non-continuous process including melt molding and two or more non-continuous forging (solid state molding) steps.
U.S. Pat. No. 6,221,075 and U.S. Pat. No. 6,692,497 describe bioabsorbable osteosynthesis plate and its surgical use. The plate is made of a material that is oriented uni- and/or biaxially and is substantially rigid and deformable at temperatures below the glass transition temperature of the material.
Uniaxially oriented plate material has good mechanical strength in the tensile mode but it is strongly anisotropic so that longitudinal splitting of the material is a risk, when mechanical forces are stressing the plate. On the other hand, biaxial orientation reduces the good tensile strength and shear strength of uniaxially oriented material.
Therefore there is a need of bioabsorbable (bioresorbable or biodegradable) oriented material or plate, which has the good shear load carrying capacity and good resistance against longitudinal splitting and which material or plate can be deformed below Tg of the material, yet dimensionally stable at said temperatures.
A need also exists for a material and plate, which in addition to aforementioned properties is strong, tough, and does not produce a substantial inflammatory response.
A need also exists for such a material and plate, which further can be deformed, yet is dimensionally stable, in operation room conditions, to facilitate the shaping of the plate.
A need also exists for such a material and plate, which has all the above-mentioned properties and can be deformed, yet is dimensionally stable in operation room conditions (in the first thermochemical state) to allow its fixation on bone without distortion of the configuration of the bone fragments to be fixed and which shaped plate is also dimensionally stable in tissue conditions (in the second thermochemical state), when fixed on bone surface to facilitate non-problematic bone fracture healing.
A need also exists for a bioabsorbable material and plate of the aforementioned type, which can be manufactured rapidly and effectively.